1. Field of the Invention
The present invention relates to a liquid crystal display device and more particularly, to an in-plane switching mode liquid crystal display device and a method for fabricating the same.
2. Description of the Related Art
A liquid crystal display (LCD) device is commonly used as a flat panel display device because it provides high quality images and consumes low power. The LCD device is formed by attaching face to face a thin film transistor array substrate and a color filter substrate with a uniform gap between the two substrates, and forming a liquid crystal layer between the two substrates. Pixels are arranged on the thin film transistor array substrate in a matrix. A thin film transistor, a pixel electrode and a capacitor are formed in each pixel region. A common electrode, an RGB color filter, and a black matrix are formed on the color filter substrate. An electric field is generated between the common electrode and the pixel electrode and applied to the liquid crystal layer. The RGB color filter implements colors.
Alignment films are formed at facing surfaces of the thin film transistor array substrate and the color filter substrate. The alignment films are rubbed to arrange the liquid crystal layer in a specific direction. Liquid crystals rotate due to a dielectric anisotropy when the electric field is applied to the liquid crystal layer between the pixel electrode formed in each pixel region and the common electrode formed on the entire surface of the color filter substrate. The rotation of the liquid crystals causes individual pixel regions to transmit or block light for displaying letters or images.
However, such a twisted nematic mode LCD device has a narrow viewing angle. Therefore, research is actively ongoing in developing an in-plane switching (IPS) mode LCD device to solve the viewing angle problem. In an IPS mode LCD device, liquid crystal molecules are aligned nearly parallel to a substrate.
FIG. 1A is a plane view of a pixel region of the related art IPS-LCD device. FIG. 1B is a cross-sectional view taken along line I-I′ of FIG. 1A. Referring to FIGS. 1A and 1B, a gate line 1 and a data line 3 are arranged horizontally and vertically on a transparent first substrate 10 to define a pixel region. Although N gate lines 1 and M data lines 3 cross each other to define N×M pixels in an actual LCD device, only one pixel is illustrated in the drawing to simplify the description.
A thin film transistor 9, including a gate electrode 1a, a semiconductor layer 5 and source/drain electrodes 2a and 2b, is disposed at the crossing of the gate line 1 and the data line 3, and the gate electrode 1a and the source/drain electrodes 2a and 2b are respectively electrically connected to the gate line 1 and the data line 3. A common line 4 is arranged parallel to the gate line 1 within the pixel region, and at least one pair of electrodes switching liquid crystal molecules, namely, a common electrode 6 and a pixel electrode 7 are arranged parallel to the data line. The common electrode 6 is formed simultaneously with the gate line 1 and electrically connected to the common line 4. A gate insulation layer 8 is formed over an entire substrate 10 including the common electrode 6. The pixel electrode 7 is formed on the insulation layer 8 simultaneously with the source/drain electrodes 2a/2b and electrically connected to the drain electrode 2b of the thin film transistor 9. A passivation film 11 is formed over the entire substrate including the source/drain electrodes 2a and 2b. A pixel electrode line 14 overlapping the common line 4 and electrically connected to the pixel electrode 7 forms a storage capacitance (Cst) with the insulation film 8 interposed between the common line 4 and the pixel electrode line 14.
A black matrix 21 for preventing light leakage to the thin film transistor 9, the gate line 1 and the data line 3, and a color filter 23 are formed on a second substrate 20, and an overcoat film (not shown) is applied on the color filter 23 to flatten the color filter 23. Alignment films 12a and 12b are applied to facing surfaces of the first substrate 10 and the second substrate 20. The alignment films 12a and 12b determine an initial alignment direction of the liquid crystals. A liquid crystal layer 13 is formed between the first substrate 10 and the second substrate 20 in order to control transmittance of light by a voltage applied to the common electrode 6 and the pixel electrode 7.
The related art IPS-LCD device having the above-described structure improves viewing angle because the common electrode 6 and the pixel electrode 7 are placed on the same substrate and generate an in-plane electric field. However, the IPS-LCD device has the following problems. Because the common electrode 6 and the pixel electrode 7, which is formed of an opaque metal, are placed within the pixel region where an image is displayed, and brightness degraded.
Furthermore, an in-plane electric field is not normally formed within the pixel region because of signal interference between the data line 3 and the pixel electrode 7. The aperture is reduced when the width of the common electrode 6 adjacent to the data line 3 is increased to prevent signal interference between the data line 3 and the pixel electrode 7. Specifically, a data signal supplied to the data line 3 may affect the pixel electrode 7 adjacent to the data line 3, which causes a distortion of the electric field. Particularly, if the pixel electrode 7 is formed parallel to the data line 3 as shown in FIG. 1A, the data line 3 causes strong signal interference with the pixel electrode 7. To shield the signal interference from the data line 3, a width of the outermost common electrode 6 adjacent to the data line 3 is increased. Accordingly, the aperture ratio is reduced.